Prior Art 1.
Processes and devices like that are for example known from DE-A 44 46 857. In these usually one material pressure sensor is provided in the nozzle and one in the mould cavity, the results of which certainly are input in the control, as for the rest, however, do not correlate with each other. As soon as the measured value of the internal mould pressure, for example after sealing the moulding, is not available anymore, this also is not available as input parameter anymore.
In order to determine the closing force more precisely, in DE-A 39 39 728 two pressure measuring systems are provided, one of which is assigned to the mould cavity and one to the mould parting plane. For the closing force determination there is, however, only one sensor available.
Another possibility for determining the force in a hydraulic injection molding machine according to EP-A 644 030 consists in detecting the forces on both sides of the injection piston and to subtract them one from the other. Such a force measurement, however, does not consider the losses which might occur between the feeding means and the injection piston.
For determination of the internal mould pressure, according to EP-A 130 769, several sensors can be provided in both mould parts, which however are not designed for measuring different force ranges.
Mostly static sensors are provided for determination of the injection forces arising during the injection cycle in the bedding area of the feeding means, as for example, a feed screw, in EP-B 331 735 for example. The measured values gained from that usually are input in a force- or pressure-dependant injection control, provided that measured values of the internal mold pressure can be detected sufficiently or due to the material used can be detected reliably from a determined time during the injection cycle.
From EP-B 436 732 it is furthermore known to arrange a pressure sensor for the same purpose in the area of the feed screw itself.
The known devices, however, have the problem, that the injection force in an injection molding machine during the real injection ranges in a very high area of mostly several tons. On the other hand during the dosing process, on the material preparation stage, the resolution should range only in the kilogram area. Static force transducers, however, now can measure continuously starting from a zero point up to the high forces without loosing their zero point, but they do not dispose, just in the lower area, of the desired resolution or, if they do so, they are not suited to measure high forces.
In order to solve this problem, nowadays expensive so-called dynamic force transducers are used as sensors. However, they have the disadvantage, that their reliable application requires, that a reset must be possible sometime during the course of the cycle. Without this reset the sensor drifts away and leaves its zero position. Since at no time during the injection process in an injection molding machine for plastics is a defined force zero condition reached, this reset cannot be achieved without the danger of building-up.